Birdcage coils are commonly used in NMR and MRI instrumentation to produce the RF field over the sample or the object being imaged. The birdcage coils are described, for example, in the U.S. Pat. Nos. 4,689,548 and 4,694,255. The conventional birdcage coil consists of a number of evenly spaced leg conductive elements interconnecting a pair of ring conductive elements. Each conductive element includes at least one reactive element that may be a capacitive or inductive element. There are two basic designs of birdcage coils: high-pass birdcage coils having inductive leg elements and capacitive ring elements, and low-pass birdcage coils having capacitive leg elements and inductive ring elements. There are also “band pass” or hybrid versions that use a combination of capacitive and inductive elements as leg or ring elements.
Basically the birdcage coil is a linear network of identical cells connected together so that the last cell in a ring is connected to the first cell. From the spatial point of view, each cell comprises a pair of ring elements coupled to a leg element forming a “ladder” network. When excited by RF energy, waves propagate along the network. For some particular frequencies the waves combine constructively corresponding to the resonant modes of the network. For the resonance of interest, the phase of the current in each adjacent leg is shifted by an angle φ=2π/N, and the amplitude of the current in each leg follows the cosine relationship:In=I cos(2πn/N),where N is a number of cells and n=1, 2, . . . N.
In a typical experiment one or more pulses of radio frequency (RF) magnetic field are applied to the sample or object in the probe to excite a nuclear resonance signal. This is followed by a reception period where the transmitter is silent and the receiver is activated to detect and record any response signal produced by the nuclei. In some systems the same coil or resonator is used to produce the transmit RF magnetic field and to receive the response signal of the nuclei. In other systems including the systems described here, a birdcage coil is used to excite a nuclear resonance signal and a separate coil or coils are used to detect the response signal produced by the nuclei. Residual coupling between the transmitter and receiver coils reduces the sensitivity during the receive mode. The small NMR currents in the receiver coil windings induce currents in the transmitter coil windings causing a loss in sensitivity since the power is absorbed and not available for signal detection. Direct coupling of the RF fields produced by the nuclei also induce currents in the transmitter coil causing a loss in sensitivity.
In attempt to solve the problem switching diodes were utilized to detune or disable the transmit or body coil in MRI as disclosed in the U.S. Pat. No. 4,763,076. The diodes were connected in series with the transmitter coil and must be forward biased during the transmit mode. When the diodes were forward biased by a DC current flowing from an anode to a cathode, the diodes provided a path for the RF currents. The diodes were reverse biased to detune or disable the transmitter circuit. A reverse biased diode provides RF signal isolation between its anode and cathode. Radio frequency choke coils or traps may be used in the lines for conducting the DC current to the switching diode and preventing RF currents from flowing on the lines.
A birdcage coil described in the U.S. Pat. No. 4,833,409 comprises a circuit for dynamically disabling it to allow for localized coil to receive the NMR signals. Each end ring of the birdcage coil is coupled to a shield surrounding the birdcage coil by four switchable impedance circuits equidistantly spaced around each end ring. When activated, the circuit provides a low impedance path between the coil and ground. This detunes the cells that are coupled to the impedance switch thereby affecting the tuning of the birdcage resonator. Though the tuning of the four cells that are coupled to the switch elements are affected, and the birdcage coil as a whole would no longer produce a resonance, currents are still induced in the individual cell inductive elements. In spite of the fact that these currents are not in a circuit that resonates at the NMR frequency, voltages are still induced in the loops and the resulting currents are smaller, but not zero or near zero because of the still finite impedance of the cell elements at the NMR frequency.